The present invention relates to methods of fluid loss control, diversion, and sealing using deformable particulates. More particularly, the present invention relates to methods useful in subterranean operations using malleable particulates comprising a degradable polymer combined with a plasticizer.
Providing effective fluid-loss control for well treatment fluids is highly desirable. A “treatment fluid” is a fluid used in a subterranean application for any purpose. The term “treatment” as used herein does not imply any particular action by the fluid or any component thereof. Fluid-loss control materials are often added to treatment fluids to aid in fluid loss control. These are additives that are generally designed to lower the volume of a filtrate that passes through a filter medium, e.g., permeable rock or a filter cake. Most attain their fluid-loss control from the presence of solvent-specific solids, or from polymers that rely on filter cake buildup and on viscoelasticity to inhibit flow into and through the formation. A variety of fluid-loss control materials have been used and evaluated, including foams, oil-soluble resins, acid-soluble particulates, graded salt slurries, linear viscoelastic polymers, degradable polymers and heavy metal-crosslinked polymers. Their respective comparative effects are well documented.
An example of a subterranean treatment that often uses an aqueous treatment fluid is hydraulic fracturing. In a hydraulic fracturing treatment, a viscous fracturing fluid is introduced into the formation at a high enough rate to exert a sufficient pressure on the formation to create and/or extend fractures therein. The viscous fracturing fluid suspends proppant particles that are to be placed in the fractures to prevent the fractures from fully closing (once the hydraulic pressure is released), thereby forming conductive channels within the formation through which hydrocarbons can flow toward the well bore for production. In certain circumstances, a portion of the fracturing fluid may be lost during the fracturing operation, e.g., through undesirable leak-off into natural or manmade fractures present in the formation. Typically, operators have attempted to solve this problem by including a fluid loss control additive in the fracturing fluid. Many conventional fluid loss control additives permanently reduce the permeability of a subterranean formation, negatively affect the rheology of the treatment fluid in which they are used, and/or reduce the rate at which the fluid is allowed to penetrate or leak off into desirable locations within the subterranean formation. Moreover, while it may be desirable to control or prevent fluid loss for a given period of time, in some instances it may become desirable to later allow a treatment fluid to penetrate or leak off into that portion of the subterranean formation. Thus, costly and time-consuming operations may be required to reverse the effects of conventional fluid loss control additives on the treatment fluid and/or to restore permeability to those portions of the subterranean formation affected by the fluid loss control additives.
In addition to helping control the loss of fluid into the formation, additives may also be added to treatment fluids in order to divert the treatment toward desired areas within the formation. For example, it may be desirable to add a diverting agent toward the end of an operation treating a section of a subterranean formation such that the agent will then slow or stop the flow of further treatment fluid into that area, thus diverting later-placed fluid to other areas.
Various particulates have been used in treatment fluids as a fluid loss control agent and/or diverting agent. Particulates may be used as fluid loss control materials in treatment fluids to fill and seal the pore spaces and natural and manmade fractures in a subterranean formation or to contact the surface of a formation face or proppant pack, thereby forming a filter cake that blocks the pore spaces and natural and manmade fractures in the formation or proppant pack.
The use of particulates in fluid loss control presents several challenges. For example, if the sizes of the particulates are not optimized for the pore spaces and fractures, remedial treatments may be required to remove the previously-placed fluid loss control materials. Generally, particulates that have become lodged in pore spaces, fractures, or pore throats may be difficult or costly to remove. Furthermore, particulate fluid loss control materials may not be effective in low-permeability formations since the leakoff rate in those formations is not high enough to pull the particulates into the pore spaces or fractures or into contact with the surface of the formation face or proppant pack so as to block or seal off the pore spaces and fractures.
The particles of polymeric fluid loss control additives may also require a proper distribution of sizes in order to effectively fill the pore spaces. The “modality” of a polymeric material generally refers to the number of size ranges in the mixture of particles. For example, a “monomodal” polymeric material refers to a material that comprises molecules that have particle size distributions within a single range, whereas a “multimodal” polymeric material refers to a material that comprises at least two pluralities of polymer molecules having different average particle sizes. In practice, multimodal particulates are generally better at fluid loss control as it is not always possible to predict the exact sizes of the pore spaces, fractures, and the pore spaces may be heterogeneous in size. That is, a heterogeneous mixture of particles is typically better suited, as compared to a homogeneous mixture of particle sizes, to seal a range of various pore sizes and fractures that are encountered downhole. A mixture of sizes provides more effective bridging and shut off in downhole applications.
Particulates made of degradable material are especially useful due to the ease associated with their clean up. While degradable particulates may be useful for a variety of downhole operations, degradable particulates can be problematic in that they are difficult to grind into the small size necessary to prevent fluid loss. For example, degradable particulate materials such as polylactic acid often require cryogenic grinding to achieve the necessary small sizes needed to seal pore spaces and fractures. Furthermore, there are technical and physical barriers to how small polylactic acid may be ground.